In an electrophotographic image forming apparatus, a surface of a photoconductor serving as an image bearing member is charged with a charge of a predetermined polarity by discharge. The charged surface of the photoconductor is exposed to light to form an electrostatic latent image on the surface that is determined by image data. The electrostatic latent image is supplied with toner charged to the same polarity as the charge polarity to form a toner image. (Herein, toner charged to the same polarity as the charge polarity will be referred to as normally charged toner.) The toner image formed on the photoconductor is then transferred onto, for example, a recording sheet and fixed thereon by the application of heat and pressure.
After the transfer of the toner image, some of the toner having failed to be transferred remains on the surface of the photoconductor. Prior to the next charging process, therefore, the surface of the photoconductor is cleaned by cleaning members, such as a cleaning blade and a cleaning brush.
A known method of charging the surface of the photoconductor involves bringing a conductive charging roller into proximity to or contact with the surface of the photoconductor and a voltage is applied between the charging roller and the photoconductor in the proximity or contact state to charge the surface of the photoconductor. This arrangement has the advantage of reducing ozone production and power consumption. In recent years, therefore, such a charging device has been put to practical use in the image forming apparatus as described above.
However, if the post-transfer residual toner remaining on the surface of the photoconductor after the transfer of the toner image is not completely removed in the cleaning process and reaches an area in which the toner comes into proximity to or contact with the charging roller, the post-transfer residual toner adheres to the charging roller. The post-transfer residual toner includes so-called oppositely charged toner, which is not charged to the normal polarity but is charged to the opposite polarity. Although the normally charged toner electrostatically repels the charging roller, and thus hardly adheres to the surface of the charging roller, by contrast the oppositely charged toner electrostatically attracts the charging roller, and thus easily adheres to the surface of the charging roller. Moreover, in addition to the oppositely charged toner, any dust such as paper powder, for example, charged to electrostatically attract the charging roller adheres to the charging roller.
Further, in recent years, with an increase in demand for high-quality and high-definition images, a toner consisting of small-diameter spherical particles has come to be used in the development process. Such toner is designed to adhere more closely to the electrostatic latent image. By the same token, however, small-diameter spherical particles easily pass under the cleaning blade in the cleaning process, and thus tends to cause a cleaning failure. To prevent the toner not removed in the cleaning process and remaining on the photoconductor from adhering to the charging roller and obstructing uniform charging of the surface of the photoconductor, therefore, the surface of the charging roller should be thoroughly cleaned.
Related-art cleaning members for cleaning the charging roller include, for example, a sponge member formed of a material such as polyurethane foam and polyethylene foam, and a brush roller (as disclosed, for example, in Japanese Patent Application Publication No. 05-297690). Related-art cleaning members further include a cleaning roller formed by a metal shaft and a sponge or brush provided around the outer circumference of the shaft (as disclosed, for example, in Japanese Patent No. 3695696). Such a cleaning roller is pressed against the surface of the charging roller, and is rotated with the rotation of the charging roller, thereby removing deposits such as toner from the charging roller. Methods of pressing the cleaning member against the charging roller include a biasing method using biasing members such as springs and a biasing method using the weight of the cleaning member.
According to the biasing method using springs, it is possible to reliably press the cleaning member against the charging roller by adjusting the spring load. The method, however, uses biasing members such as springs, thus increasing the number of components. Further, if the cleaning member is left pressed against the charging roller for an extended period of time, the cleaning member tends to be permanently deformed, resulting in deterioration of cleaning performance.
With the biasing method using the weight of the cleaning member, the cost is reduced owing to fewer components. The pressure applied to the charging roller is adjusted by adjustment of the weight of the cleaning member. Specifically, to increase the contact pressure, the diameter of the shaft of the cleaning roller is increased. With demand in recent years for further reduction in both device size and cost, however, the cleaning roller is desired to be as thin as possible. Such a reduction in diameter of the cleaning roller results in insufficient contact pressure applied to the charging roller. As a result, the cleaning roller slips or bounces on the charging roller, and the cleaning effect is reduced.
To prevent the cleaning roller from bouncing, the cleaning device can be configured such that, in the inner wall of a hole formed in each of shaft supporting members for holding the cleaning roller, the contact resistance is reduced on the side close to the charging roller and increased on the side far from the charging roller (as disclosed, for example, in Japanese Patent Application Publication No. 2007-193247). If the contact resistance is increased enough to prevent the bouncing, however, the cleaning roller may be caught by the shaft supporting members and fail to return to the previous position after the bouncing.